1. Field of the Invention
The present invention relates to a radio communications system which establishes connection, through a radio channel, between a fixed terminal located at the home of an ordinary subscriber or at a business establishment and an exchange or a basic trunk cable, and which offers communications services such as a telephone service, an information service, a graphic service, and a multimedia service.
2. Description of the Related Art
A conventional dominant cell structure employed in the field of mobile communications such as that found in a radio system is a sector cell configuration, wherein a directional antenna is used for a site formed from a plurality of sector cells (often simply referred to as xe2x80x9csectorsxe2x80x9d). FIGS. 27 and 28 show an example of the sector cell configuration. In FIG. 27, each site is formed from three sectors. Seven sites comprising a total of 21 sectors are handled as a unit, and frequencies are allocated repeatedly on a per-unit basis (such a configuration will hereinafter be referred to simply as a xe2x80x9cfrequency repetition every three-sectors/seven-sitesxe2x80x9d). FIG. 28 shows an example in which each site is formed from six sectors. Seven sites comprising 42 sectors are handled as a unit, and frequencies are allocated repeatedly on a per-unit basis (such a configuration will be hereinafter referred to as a xe2x80x9cfrequency repetition every six-sectors/seven-sitesxe2x80x9d). In the drawings, reference numerals 91 to 97 and 98 to 103 designate sites; and 97 and 104 designate service areas, wherein areas allotted the same reference numerals are assigned the same frequency. In FIG. 27, the site 91 receives an interference wave signal of 7.7 dB emitted from fixed terminal stations disposed in the service areas 97 of the sites 92 to 96. In FIG. 28, the site 98 receives an interference wave signal of 10.5 dB emitted from the fixed terminals disposed in the service areas 104 of the sites 99 to 103.
FIG. 29 is a plan view similar to a plan view of sector zones provided in Japanese Patent Application Laid-Open No. Hei-8-193556 entitled xe2x80x9cRadio System,xe2x80x9d showing the relationship between frequencies used among adjacent sites and the layout of beams emanated from antennas. In the drawing, reference numerals 105 to 107 designate service areas of individual antennas, and areas allotted the same reference numeral are assigned the same frequency. Reference numerals 108 and 109 designate sites; and 110 designates a fixed terminal station. In a radio communications system, radio communication is established between a site, such as a subscriber radio system, and a plurality of fixed terminal stations disposed in a cell covered by the site. As can be seen from the cell layout shown in FIG. 29, in each of the plurality of adjacent sites frequencies are assigned to the cells in the same sequence. Further, the centers of the beams emanated at the same frequency from the antennas are offset from one site to another adjacent site by an amount approximately equal to the beam angle.
In a conventional radio communications system, a certain district is divided into a plurality of cells, and a site is disposed at the center of each cell. Radio communication is established between the site and a plurality of fixed terminals disposed in the cell where the site is located. The cells are assigned frequencies in the same sequence, and the centers of beams emanated from the antennas are offset from one site to another adjacent site by an amount approximately equal to the beam angle. As shown in FIG. 29, a fixed terminal 110 receives a radio wave signal from the site 109, in addition to a radio wave signal from the site 108, thus receiving interference wave signals of the same frequency. At this time, a receiving carrier-to-interference (C/I) ratio, which is a ratio of carrier wave signal power to interference wave signal power is 5.2 dB, thus inducing considerable radio interference.
The present invention has been made to solve the above problems with the prior art, and therefore an object of the present invention is to provide a radio communications system which diminishes the level of radio interference by ensuring a sufficient C/I ratio and required frequencies.
According to a first aspect of the present invention, there is provided a radio communications system which establishes radio communication between arbitrary sites and fixed terminal stations which are directional and disposed in a cell centered on the site, wherein
each of the sites is provided with three antennas of equal horizontal beam width and whose orientations are horizontally offset from one another;
at each site, the beams emanated from the individual antennas, as a whole, cover all horizontal directions;
the antennas assigned the same frequency as that of the site are arranged in the same sequence;
a cell group comprises three cells, in which the centers of beams emanated from the antennas assigned the same frequency are offset from one site to another site by an amount approximately equal to the beam width;
the cells are adjacent to one another;
the cell group comprises a plurality of groups of different frequencies and constitutes a minimum unit area to be repeated; and
the minimum unit areas are repeatedly arranged in longitudinal and lateral directions.
According to a second aspect of the present invention, there is provided a radio communications system which establishes radio communication between arbitrary sites and fixed terminal stations which are directional and disposed in a cell centered on the site, wherein
each of the sites is provided with xe2x80x9cnxe2x80x9d (where xe2x80x9cnxe2x80x9d represents a positive integer equal to or greater than 4) sector antennas which are equal in horizontal beam width and whose orientations are horizontally offset from one another;
at each site the beams emanated from the individual antennas, as a whole, cover all horizontal directions;
frequencies are arranged such that sectors within a cell are assigned the same frequency every xe2x80x9cmxe2x80x9d sectors (where xe2x80x9cmxe2x80x9d represents a positive integer of equal to or greater than 2, and m less than n);
the antennas assigned the same frequency as that of the site are arranged in the same sequence;
a cell group comprises xe2x80x9cmxe2x80x9d cells, in which the centers of beams emanated from the antennas assigned the same frequency are horizontally offset from one site to another site by an amount approximately equal to the beam width;
the cells are adjacent to one another;
the cell group comprises a plurality of groups of different frequencies and constitutes a minimum unit area to be repeated; and
the minimum unit areas are repeatedly arranged in longitudinal and lateral directions.
According to a third aspect of the present invention, there is provided a radio communications system which establishes radio communication between arbitrary sites and fixed terminal stations which are directional and disposed in a cell centered on the site, wherein
each of the sites is provided with three antennas which are equal in horizontal beam width and whose orientations are horizontally offset from one another;
at each site, the beams emanated from the individual antennas, as a whole, cover all horizontal directions;
the antennas assigned the same frequency as that of the site are arranged in the same sequence;
a cell group comprises three cells, in which the centers of beams emanated from the antennas assigned the same frequency are offset from one site to another site by an amount approximately equal to the beam width;
the cells are arranged such that a cell having an antenna assigned different frequency is interposed between adjacent cells;
the cell group comprises a plurality of groups of cells assigned different frequencies and constitutes a minimum unit area to be repeated; and
the minimum unit areas are repeatedly arranged in longitudinal and Lateral directions.
According to a fourth aspect of the present invention, there is provided a radio communications system which establishes radio communication between arbitrary sites and fixed terminal stations which are directional and disposed in a cell centered on each site, wherein
each of the sites is provided with xe2x80x9cnxe2x80x9d (where xe2x80x9cnxe2x80x9d represents a positive integer equal to or greater than 4) sector antennas which are equal in horizontal beam width and whose orientations are horizontally offset from one another;
the beams emanated from the individual antennas, as a whole, cover all horizontal directions;
frequencies are arranged such that sectors within a cell are assigned the same frequency every xe2x80x9cmxe2x80x9d sectors (where xe2x80x9cmxe2x80x9d represents a positive integer of equal to or greater than 2, and m less than n);
the antennas assigned the same frequency as that of the site are arranged in the same sequence;
a cell group comprises xe2x80x9cmxe2x80x9d cells, in which the centers of beams emanated from the antennas assigned the same frequency are offset from one site to another site by an amount approximately equal to the beam width;
the cells are arranged such that xe2x80x9cLxe2x80x9d (where xe2x80x9cLxe2x80x9d designates 0 or a positive integer greater than 1) cells having an antenna assigned a different frequency is interposed between the adjacent cells;
the cell group comprises a plurality of groups of different frequencies and constitutes a minimum unit area to be repeated; and
the minimum unit areas are repeatedly arranged in longitudinal and lateral directions.
According to a fifth aspect of the present invention, the radio communications system as defined in any one of the first through fourth aspects is further characterized by that the minimum unit areas are repeatedly arranged in the vertical direction so as to become horizontally offset from one another.
According to a sixth aspect of the present invention, the radio communications system as defined in any one of the first through fourth aspects is further characterized by that the minimum unit areas are repeatedly arranged such that a column of minimum unit areas becomes vertically offset from another column of minimum unit areas.